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Bays and estuaries rely on freshwater inflows to maintain the salinity gradient necessary to sustain their fisheries. Reduced freshwater inflows, particularly during summer seasons, can be detrimental to the health of these systems. Despite an extensive network of streamflow gauges in the U.S., many coastal watersheds remain ungauged. The primary objective of this study was to develop methods to build a water budget for an ungauged watershed using limited data to determine the watershed contribution of freshwater to an at-risk bay system. This method was developed and tested for Big Boggy Creek, which flows into East Matagorda Bay (EMB), Texas. The streamflow into and out of Big Boggy Creek was quantified at key upstream and downstream sites. Over the summertime study period, we found average monthly freshwater inflows of 244 megaliters (ML). A simple inflow decision tool was developed to assist resource managers in estimating freshwater inflows during the summer months in the study area. Two recommendations are provided to increase freshwater inflows to EMB, with the most approachable option being purchasing water from a regional river authority. The framework developed herein can be modified and applied to ungauged watersheds to budget, model, and predict freshwater inflow contributions. [ABSTRACT FROM AUTHOR]

It has become clear that estuaries with low rates of freshwater inflow are an important but overlooked sphere of estuarine science. Low-inflow estuaries (LIEs) represent a major class of estuary long downplayed because observations do not fit well in the dominant estuary paradigm, which was developed in perennially wet climates. Rather than being rare and unusual, it is now evident that LIEs are common globally and an alternate estuary paradigm within the idea of an estuary as the place where a river meets the sea. They are found mostly in areas with arid, semi-arid, or seasonally arid climates, but LIE phenomena are also found in estuaries along mountainous coasts with small watersheds and short-tailed hydrographs. Inflows can be defined as "low" relative to basin volume, tidal mixing, evaporative losses, or wave forcing at the mouth. The focus here is on common physical phenomena that emerge in low-inflow estuaries—how low river flow is expressed in estuaries. The most common is hypersalinity (and the associated potential for inverse conditions), which develops where there is a net negative water balance. However, in small microtidal estuaries, low inflow results in mouth closure even as a positive water balance may persist, accounting for extreme stratification. Attention is also given to the longitudinal density gradient and the occurrence of thermal estuaries and inverse estuaries. Finally, ocean-driven estuaries are highlighted where marine subsidies (nutrients, particulates) dominate watershed subsidies. While climate change is altering freshwater inflow to estuaries, locally driven changes are generally more important and this presents an opportunity to restore estuaries through restoring estuarine hydrology. [ABSTRACT FROM AUTHOR]

Low-inflow estuaries are naturally more susceptible to anthropogenic stressors, compared to well flushed systems, with excessive nutrient loading posing a particular threat. This study investigated the benthic diatom community structure of two eutrophic, microtidal estuaries impacted by daily wastewater effluent discharges. It was hypothesised that the community structure would be similar between the warm-temperate Hartenbos and subtropical uThongathi estuaries due to disproportionally high dissolved inorganic nitrogen (H: 38 kg DIN d−1; uT: 67.5 kg DIN d−1) and phosphorus (H: 22 kg DIP d−1; uT: 29.7 kg DIP d−1) inputs from wastewater treatment works (WWTWs). Taxa tolerant of high nutrient conditions proliferated in both systems. However, the dominant taxa differed with the brackish Halamphora coffeaeformis species occurring in the Hartenbos Estuary and the freshwater Navicula rostellata, Sellaphora pupula and Navicula gregaria species in the uThongathi Estuary. The overall benthic diatom diversity in both systems was low (H' > 0 but ≤ 1.5) and indicative of a degraded health state. Temporal differentiation driven by salinity was evident in the Hartenbos Estuary, while changes in community structure were limited to periods of increased river inflow in the uThongathi Estuary. Therefore, while the trophic status of the dominant taxa was determined by the nutrient stress (primary stressor), changes in salinity and river inflow (secondary stressors) shaped the distinct community assemblages observed in each estuary. This study provides insight into the impact of similar anthropogenic-induced pressures in different biogeographical regions and the importance of managing towards a natural dynamic state of microtidal, low-inflow estuaries. [ABSTRACT FROM AUTHOR]

Changes in freshwater flow to estuaries can cause a suite of ecosystem impacts including eutrophication and alterations to plant communities, zooplankton populations, and other biota. In southwest Florida, historical manipulation of freshwater flow due to development, canals, and drainage ditches is pervasive. Because there are estuaries with reduced, increased, and relatively natural freshwater flow, this region presents an ideal system to study how these changes relate to downstream fish abundances. We used a 20-year trawl dataset focused on juvenile and small-bodied fish from three mangrove-lined sub-estuaries with contrasting flow conditions in southwest Florida's Ten Thousand Islands to identify important environmental and temporal variables influencing fish populations. We used generalized additive models to investigate total fish abundance, species richness, diversity, and the abundances of 23 ecologically, recreationally, or commercially important species and describe their relationships with important environmental and temporal variables. While salinity and temperature had species-specific relationships with fish abundances, seasonality, interannual variability, and sub-estuary were more closely related to fish than salinity and temperature in most cases. Only 8 of 23 species responded most strongly to temperature, and only 1 species responded most strongly to salinity. This suggests that for most species in our study, temporal factors such as timing of spawning and recruitment variability had stronger relationships with the structure of fish populations than changes in freshwater flow. This work quantified how changes in freshwater flow, using salinity as a proxy, may relate to downstream fish abundances and therefore the potential implications of planned watershed restoration that is part of the Comprehensive Everglades Restoration Plan. [ABSTRACT FROM AUTHOR]

Buoyant coastal current induced by riverine outflow often propagates along the coast in the direction of coastal trapped wave. However, it is uncertain how the buoyant coastal current interacts with irregular coastal bathymetry. Measured surface salinity distributions indicate that the buoyant coastal current, which is induced by the Changjiang River plume, frequently penetrates offshore around the curved headlands in the Zhoushan Islands. This signal can also be seen in satellite chlorophyll data, reflecting its importance in cross‐shelf exchange both physically and ecologically. To understand the variation and underlying dynamic mechanisms of these penetrating fronts, realistic and idealized numerical experiments were conducted. The results indicated that the penetrated water salinity was about 30–32 psu, with a horizontal penetration scale of tens of kilometers. Curvature and self‐organizing map analyses suggested that it was a frequent event with an approximately 2‐week spring‐neap cycle from generation to decay. It more likely occurred in transition seasons such as spring and autumn, owing to the runoff accumulation and gentle wind conditions. Smoothing the topography would substantially eliminate the penetrating fronts. Idealized model experiments suggested that headland‐induced river plume penetration was a type of curved topography‐induced current separation. The criterion for this to occur is u/fRc > 1, in which u is the current velocity, f is the Coriolis parameter, and Rc is the curvature radius. In this case, the headland is too sharp that the Coriolis force could not balance the centrifugal force. Once penetrated, the buoyant water was further transported by the ambient Taiwan Warm Current. Plain Language Summary: Low‐salinity riverine freshwater flows out of an estuary, mixes with high‐salinity ambient seawater and propagates along the coast, forming buoyant coastal currents. It is not always steady in a realistic coastal environment, especially in the upper layer, which is modulated by surface wind and tidal currents. This results in frontal instability, otherwise called "penetrating fronts." Previous studies have noted the frontal disturbance of buoyant coastal currents, and some explanations have been given for its origin. However, few attentions have been paid on the effect of irregular coastal topography like the headland bathymetry. This study investigated this topic by studying the interaction between the Changjiang River plume and the headland topography around the Zhoushan Islands as an example. Through a series of well‐designed numerical experiments using both realistic and ideal models, the results showed that headland‐induced current separation would happen if the headland was sharp enough, which was the main reason that the Changjiang River plume penetrated around two headlands. In addition, ambient shelf current and appropriate runoff and wind conditions were prerequisites for it. The findings highlighted the effect of complex coastal topography on buoyant coastal currents and promoted our understanding on coastal biological processes associated with such cross‐front exchanges. Key Points: Penetrating fronts of the Changjiang River plume occurred around the headland bathymetry in the Zhoushana IslandsSeasonal and spring‐neap variations of penetrating fronts were significant under different wind and runoff conditionsThe main mechanism for the penetration was that the Coriolis force could not balance the centrifugal force, resulting in separation [ABSTRACT FROM AUTHOR]

The contents of total mercury (THg) and methylmercury (MMHg) were measured in Nansi Lake and its inflow river estuaries to investigate their spatial distribution characteristics and the geochemical relationships between THg, MMHg, and methylation rate (%MMHg) with mineral phases of the sediment. The contents of THg, MMHg, and %MMHg in the lake sediment were lower than those in the inflow river estuary sediment. Inputting of inflow rivers was primary source to THg and MMHg in Nansi Lake. The THg contents in the sediment decreased from the upper to the lower lake, while the MMHg contents indicated the opposite trends of spatial variations. The THg was significantly positively correlated with MMHg and %MMHg in estuarine sediment but has a significant negative correlation with MMHg and %MMHg in lake sediment. Total organic carbon (TOC), acid volatile sulfide (AVS), and clay have great influence on the distribution of THg in sediment. Methylmercury in the estuary sediment was positively correlated with THg, AVS, TOC, and clay, while these relationships were not revealed in the lake sediment. Cluster and principal component analysis results showed that THg, clay, and TOC were derived from the same sources in the sediment of Nansi Lake. It implied that for improving the water environmental quality of lakes, the environmental management of rivers entering the lake should be strengthened. [ABSTRACT FROM AUTHOR]

Major threats to South African estuaries include freshwater flow alterations, urbanisation, habitat transformation, deteriorating water quality, increased biological invasions and resource over-exploitation. Although South African estuary restoration is in its infancy, important lessons have been learnt through past efforts undertaken in some of the most impacted systems. This review presents case studies focused on hydrological and physical restoration actions, while recognizing the need to implement urgent remedial measures to restore the populations of targeted estuary-associated marine fish species. Common interventions designed to arrest declining estuary health include artificial management of estuary mouths and the use of artificial structures to restore tidal action and remediate erosion damage. Limited natural habitat restoration actions have been conducted on the subcontinent. Fortunately, estuaries are resilient ecosystems and can recover fairly rapidly if appropriate restoration interventions are successfully applied. Restoration needs to be co-ordinated through a national strategy which is currently not in place. It is particularly important to address the root cause of deterioration through active restoration and/or rehabilitation measures rather than only managing symptoms. The implementation of existing and future conservation legislation regarding the exploitation of natural resources within estuaries needs to be prioritised, so that over-exploited angling and subsistence fish species populations can recover. [ABSTRACT FROM AUTHOR]

Land use changes and mounting water demands reduce freshwater inflows into estuaries, impairing estuarine ecosystems and accelerating coastal seawater intrusion. However, determining minimum river inflows for management guidelines is hampered by a lack of ecosystem-flow link data. This study describes the development of freshwater inflow guidelines for the Wami Estuary, combining scarce river flow data, hydrological modeling, inferring natural salinity regime from vegetation zonation and investigating freshwater requirements of people/wildlife. By adopting the Building Blocks Methodology, a detailed Environmental Flows Assessment was performed to know the minimum water depth/quality seasonal requirements for vegetation, terrestrial/aquatic wildlife and human communities. Water depth requirements were assessed for drought and normal rainfall years; corresponding discharges were obtained by a hydrological model (HEC-RAS) developed for the river channel upstream of estuary. Recommended flows were well within historically occurring flows. However, given the rapidly increasing water demand coupled with reduction in basin water storage due to deforestation/wetland loss, it is critical to ensure these minimum flows are present, without which essential ecosystem services (fisheries, water quality, mangrove forest resources and wildlife/tourism) will be jeopardized. The EFA process is described in painstaking detail to provide a reference for undertaking similar studies in data-poor regions worldwide. [ABSTRACT FROM AUTHOR]

Variation in freshwater flow into estuaries can profoundly alter abundance of estuarine organisms through a variety of mechanisms. In the San Francisco Estuary, California, an annual abundance index of juvenile longfin smelt Spirinchus thaleichthys has varied by ~ 100-fold over the range of flow, and over the last five decades the index has declined by over 100-fold. The unknown mechanisms for variation with flow may include removal of larvae by freshwater diversions during low-flow periods. Using data from larval trawl surveys during January–March 2009–2020, we estimated larval population size, its response to freshwater flow, and losses of larvae to freshwater diversions. Population size was estimated by a Bayesian hierarchical model linking a process model, with salinity and water clarity as covariates, to an observation model representing catch by a negative binomial distribution. Population size averaged across surveys within years—an index of the number of larvae produced—decreased over the study period from ~ 109 to 108 larvae. Population size was unrelated to freshwater flow in the year of hatching but positively related to the subsequent juvenile abundance index. Thus, the mechanisms underlying the strong variability in the annual abundance index of longfin smelt with freshwater flow are constrained to occur after March. Estimated proportional losses to water diversions accumulated over the period of vulnerability averaged 1.5% of the population, too low to measurably influence population dynamics. [ABSTRACT FROM AUTHOR]

Understanding the impact of anthropogenically altering freshwater flow to estuaries is a growing information need for coastal managers. Due to differences in watershed development, drainage canals, and water control structures, the Ten Thousand Islands area of southwest Florida provides an ecosystem-scale opportunity to investigate the influence of both more, and less, freshwater flow to coastal bays compared to locations with more natural hydrology. Bottom trawl and water quality data spanning 20 years were used to investigate how environmental and hydrological differences among three bays affect community structure of small estuarine fishes. Relationships between fish community structure and salinity and temperature variables were evaluated over timescales from 1 day to 3 months prior to each trawl. Longer-term aspects of temperature (i.e., 2–3 months) exhibited the highest correlations in all bays, suggesting that spawning cycles are the main cause of seasonal changes in fish communities, rather than differences in freshwater flow. Despite major contrasts in watershed manipulation and the seasonal salinity of one bay being much less than the others, the bays differed primarily based on relative abundances of more common species rather than due to unique suites of species being present. Truly freshwater conditions were never detected, and high salinity conditions were experienced in all bays during dry seasons. This likely prevents a community shift to freshwater species. The range in flow characteristics among bays and general similarity in fish communities suggest that conditions will remain within the tolerance of most fishes in all three bays following restoration to more saline conditions. [ABSTRACT FROM AUTHOR]

Global changes in precipitation patterns have increased the frequency and duration of flooding events. Freshwater inflows into estuaries reduce salinity levels and increase nutrient inputs, which can lead to eutrophication and impaired water quality. Oysters are important ecosystem engineers in coastal environments that are vulnerable to co‐occurring environmental stressors associated with freshwater flooding events. Successful recruitment is necessary to maintain adult oyster populations, but early life stage responses to multiple stressors are not well understood. Flood‐associated stressor conditions were observed near oyster habitats at multiple locations across the northern Gulf of Mexico during peak recruitment months in the spring and summer of 2021. In the laboratory, we examined the interactive effects of acidification, hypoxia, and low salinity on larval and juvenile life stages of the eastern oyster (Crassostrea virginica) to better understand the impact of flooding events on oyster development and survival. Salinity stress in isolation reduced larval growth and settlement, and decreased survival and growth at the juvenile stage. Hypoxia was more stressful to oyster larvae than to juveniles, whereas low pH had negative effects on juvenile growth. There were no synergistic effects of multiple flood‐associated stressors on early oyster life stages and effects were either additive or predicted by the salinity stress response. The negative impacts of flooding disturbances on recruitment processes in benthic populations need to be considered in restoration planning and flood control mitigation strategies as the frequency and intensity of extreme freshwater events continue to rise worldwide. [ABSTRACT FROM AUTHOR]

Sustainability solutions to water quality management problems, many generated by human activities, require knowledge and understanding of circulation patterns and transport rates that govern water quality conditions. This research demonstrates approaches to frame and implement estuary investigations with the purpose of quantifying hydrodynamics influencing estuary water pollution residence time and transport rates governed by freshwater flows and estuary tidal dynamics. The study area is a coastal location with connected estuaries in the Gulf of Maine, USA, near Acadia National Park where complex coastal morphometry controls on estuary hydrodynamics and pollution problems have been observed. A Lagrangian particle tracking study is implemented to examine particle transport patterns and timescales to identify the conditions and mechanisms that influence them. Conservative virtual particles are released and tracked in hydrodynamic simulations over month-long periods with a range of conditions specified by freshwater flow input, tide range, and varied density gradients. Results show that residual eddy size surpasses the tidal excursion length and transport behavior is controlled by the residual flow rather than the tide in wide and geometrically simple estuaries with a simple bed profile and few complications from localized geomorphological features. Estuaries with similar characteristics are pre-disposed to suppression of the estuary water density gradient, a primary driver of residual flow, producing an order of magnitude increase in residence time under conditions comparable to those simulated by our analysis. Narrow channels and the presence of complex geomorphic features such as islands and constrictions limit the length of residual eddies and increase the tidal excursion. Transport timescales in estuaries with these conditions are more sensitive to variations in the tidal range than to changes in residual flow. Explorations of the relations between residual eddy length estimated as the estuary aspect ratio and a normalized tidal excursion provide a practical way for managers to identify controls on particle transport and pollution dynamics in estuaries and determine appropriate management actions to monitor and respond to coastal water pollution problems. [ABSTRACT FROM AUTHOR]

• Strong seasonality in nutrient (DIP and DIN) concentration and flux, with clear changes in response to streamflow drought. • Dynamically changing correlations between streamflow and nutrient (DIP and DIN) flux throughout different drought phases. • Contrasting long-term changes in DIP and DIN corresponding to different streamflow regimes—high and low flows. Elongated periods of low flow conditions, which can be termed as streamflow droughts, influence the nutrient (e.g., nitrogen and phosphorus) balance in estuarine systems. Analyzing temporal trends of nutrient fluxes into such systems under different streamflow regimes can complement the understanding about the dynamic evolution of streamflow droughts and their impacts on nutrient levels. The objective of this paper was to evaluate how dynamic evolution of streamflow droughts (from low flow conditions) affects the inorganic nutrient flux in a tropical estuarine system. We analyzed a 20-year time series of streamflow data together with the concentrations of two nutrient parameters—dissolved inorganic phosphorus (DIP) and dissolved inorganic nitrogen (DIN)—in the Lower Apalachicola River that drains into Apalachicola Bay in northeastern Gulf of Mexico, Florida. Our findings revealed that droughts affect the seasonal patterns and fluxes of both DIP and DIN. We also observed post-drought flushing patterns in DIP and contrasting changes in DIP and DIN fluxes in the long-term (20 years here) under different streamflow conditions. Dynamically changing correlations between the streamflow and the fluxes were found throughout different phases of droughts. In the long-term (from 2003 to 2021), the DIP flux in high flows increased by 35.3%, while the flux decreased by 15.7% in low flows. Conversely, DIN flux in high flows showed a decrease of <1.2%, but an increase of <23.7% in low flows after droughts end. The insights from this study highlighted the need for effective regulation plans such as proper nutrient management against streamflow droughts to mitigate negative ecological consequences in estuarine systems such as harmful algal blooms. [Display omitted] [ABSTRACT FROM AUTHOR]

Low freshwater inflow estuaries are common worldwide, yet our understanding of phytoplankton dynamics in these systems is limited compared to river-influenced estuaries. Combining 3 years of monthly sampling data with analysis of a lower frequency, multidecadal dataset, we examined nutrient-phytoplankton dynamics in Baffin Bay-Upper Laguna Madre, TX (USA), a low-inflow lagoonal estuary. During a low rainfall, high salinity period (corresponding with non-El Niño conditions), phytoplankton community biovolume was high and consisted of a near monoculture of the harmful "brown tide" organism, Aureoumbra lagunensis. With the onset of El Niño conditions, rainfall increased and salinity decreased. Other phytoplankton groups became more prevalent, namely diatoms and the mixotrophic ciliate, Mesodinium sp., while prevalence of A. lagunensis declined. Although El Niño conditions corresponded with increased inorganic nitrogen concentrations, this did not lead to a near-term (weeks-months) increase in phytoplankton biovolume, indicating that the stimulatory effects of nutrient pulses may have been countered by the decreased residence times associated with increased inflow. Overall, results demonstrate that low freshwater inflows, as well as high residence times and salinities, can lead to increased phytoplankton biomass and decreased phytoplankton diversity, despite relatively low external nutrient loadings. With future expansion of arid/semiarid regions and/or increasing human freshwater demands, Baffin Bay and similar systems may experience lower inflows, more pronounced hypersalinity, and presumably less diverse phytoplankton communities, potentially dominated by harmful taxa as seen here. [ABSTRACT FROM AUTHOR]

Accurate estimates of freshwater flow to the San Francisco Estuary are important in successfully regulating this water body, in protecting its beneficial uses, and in accurately modeling its hydrodynamic and water-quality transport regime. For regulatory purposes, freshwater flow to the estuary is not directly measured; rather, it is estimated from a daily balance of upstream Delta inflows, exports, and in-Delta water use termed the net Delta outflow index (NDOI). Field research in the 1960s indicated that NDOI estimates are biased low in summer-fall and biased high in winter-spring as a result of conflating Delta island evapotranspiration estimates with the sum of ungauged hydrologic interactions between channels and islands referred to as net channel depletions. In this work, we employed a 50-year observed salinity record along with gauged tidal flows and an ensemble of five empirical flow-salinity (X2) models to test whether a seasonal bias in Delta outflow estimates could be inferred. We accomplished this objective by conducting statistical analyses and evaluating whether model skill could be improved through seasonal NDOI flow adjustments. Assuming that model residuals are associated with channel depletion uncertainty, our findings corroborate the 1960s research and suggest that channel depletions are biased low in winter months (i.e., NDOI is biased high) and biased high in late summer and early fall months (i.e., NDOI is biased low). The magnitude of seasonal bias, which can reach 1,000 cfs, is a small percentage of typical winter outflow but represents a significant percentage of typical summer outflow. Our findings were derived from five independently developed models, and are consistent with the physical understanding of water exchanges on the islands. This work provides motivation for improved characterization of these exchanges to improve Delta outflow estimates, particularly during drought periods when water supplies are scarce and must be carefully managed. [ABSTRACT FROM AUTHOR]

How an estuary responds to sea level rise (SLR) is complex and depends on energy drivers (e.g., tides and river inflows), estuarine geometry (e.g., length and depth), intrinsic fluid properties (e.g., density), and bed/bank roughness. While changes to the tidal range under SLR can impact estuarine sediment transport, water quality, and vegetation communities, studies on the altered tidal range under SLR are often based on case studies with outcomes applicable to a specific site. As such, this study produced a large ensemble of estuarine hydrodynamic models (>1800) to provide a systematic understanding of how tidal range dynamics within different estuary types may change under various SLR and river inflow scenarios. The results indicated that SLR often amplifies the tidal range of different estuary types, except for short estuaries with a low tidal range at the mouth where SLR attenuates the tides. SLR alters the location of the points with minimum tidal range and overall tidal range patterns in an estuary. Variations in tidal range were more evident in converging estuaries, shallower systems, or in estuaries with strong river inflows. These findings provide an indication of how different estuary types may respond to estuaries and may assist estuarine managers and decision makers. [ABSTRACT FROM AUTHOR]

Delta Smelt, Hypomesus transpacificus, is an endangered pelagic fish native to the San Francisco Estuary. The distribution of Delta Smelt in the estuary shifts landward from low-salinity habitat to freshwater habitat before spawning. This spawning migration often coincides with the first substantial freshwater inflow to the estuary during winter. To accomplish this landward shift in distribution, Delta Smelt are believed to use the tides by swimming to faster-moving currents during flood tides and then repositioning themselves to slower-moving currents to reduce seaward movement on ebb tides. Studies have hypothesized that the swimming behavior of Delta Smelt during this period is influenced by environmental conditions such as salinity and turbidity. The details of these swimming behaviors--including the extent to which flows, salinity, and turbidity affect behaviors and distributions--are uncertain. The spawning migration is of management interest because an increase in observed counts of Delta Smelt at the South Delta water-export facilities has coincided roughly with the spawning migration in many years. In this study, we investigated a range of hypothesized swimming behaviors using a three-dimensional particle-tracking model for water year 2002 during the spawning migration, and compared the predicted distributions of Delta Smelt to distributions inferred from catch data. Our goal was to improve understanding of the influence of Delta Smelt swimming on distribution, and, ultimately, to develop a modeling tool to help management agencies identify conditions associated with entrainment losses. Predictions of Delta Smelt distributions and entrainment varied greatly among behaviors. Without swimming, Delta Smelt would be rapidly transported seaward of Suisun Bay, while continuous tidal migration would move them deep into the interior Delta. These behaviors and a simple turbidity-driven behavior model predicted distributions inconsistent with observations, while more complex behavior rules allowed improved predictions. [ABSTRACT FROM AUTHOR]

We sampled the Klang estuary during the inter-monsoon and northeast monsoon period (July–Nov 2011, Oct–Nov 2012), which coincided with higher rainfall and elevated Klang River flow. The increased freshwater inflow into the estuary resulted in water column stratification that was observed during both sampling periods. Dissolved oxygen (DO) dropped below 63 μM, and hypoxia was observed. Elevated river flow also transported dissolved inorganic nutrients, chlorophyll a and bacteria to the estuary. However, bacterial production did not correlate with DO concentration in this study. As hypoxia was probably not due to in situ heterotrophic processes, deoxygenated waters were probably from upstream. We surmised this as DO correlated with salinity (R2 = 0.664, df = 86, p < 0.001). DO also decreased with increasing flushing time (R2 = 0.556, df = 11, p < 0.01), suggesting that when flushing time (> 6.7 h), hypoxia could occur at the Klang estuary. Here, we presented a model that related riverine flow rate to the post-heavy rainfall hypoxia that explicated the episodic hypoxia at Klang estuary. As Klang estuary supports aquaculture and cockle culture, our results could help protect the aquaculture and cockle culture industry here. [ABSTRACT FROM AUTHOR]

Links between hydrologic modifications, flow and salinity regimes, and submerged aquatic vegetation (SAV) species composition and abundance were assessed with an empirical analysis of 33 years of monitoring data collected at nine sites in Florida's Caloosahatchee River Estuary (CRE). Freshwater inflows to the estuary (30-day means) were often outside the previously recommended envelope of 12.74 to 79.29 m3 s−1. Discharges from Lake Okeechobee through a synthetic hydrologic link were responsible for 43% of the above-envelope flows, but reduced the incidence of below-envelope flows by 30%. A salinity model and salinity stress indices developed for each SAV species indicated that the observed flows generated variable salinity conditions likely to harm both seagrasses and freshwater SAV in the estuary. Regression modeling of SAV abundance generally confirmed the flow and salinity responses expected for each species: Halodule wrightii and Thalassia testudinum in the lower estuary were both harmed by high-flow, low-salinity conditions, while Vallisneria americana in the upper estuary was decimated by low-flow, high-salinity conditions. There was a species-specific effect of the seasonal timing of high flows—T. testudinum was more negatively correlated with high flows in the dry season; H. wrightii in the wet season. The regression analyses also highlighted strong, year-to-year autocorrelations in SAV abundance, indicating reduced resilience after severe losses, particularly for V. americana. Large residual variation in some regression models suggested that factors other than salinity (e.g., optical water quality or grazing impacts) may also influence the system dynamics and should be incorporated in continuing research. This analysis suggests that use of artificial water management infrastructure to reduce extreme high and low flows to the Caloosahatchee and other estuaries could help maintain SAV health in light of intensifying climate variability and degraded watershed flow regulation capacity. [ABSTRACT FROM AUTHOR]

The large Temporarily Closed Seekoei Estuary has a long history of poor management. In 1969, a swimming pool complex was built in a position impinging on the mouth. The construction of a 500 m causeway across the lagoon connecting the residential townships of Paradise Beach and Aston Bay followed. Freshwater abstraction in the catchment has escalated, whereas increasing sedimentation, duration and frequency of mouth closure and hypersaline events contributed to the deteriorating health status of the estuary. Pedestrian and vehicle numbers crossing the causeway are high, particularly among Paradise Beach residents who need to access schools, medical services, work premises, business and shopping facilities. Causeway maintenance is ongoing, whereas winter storm surges deposit large volumes of sand and seawater in the estuary. The causeway becomes submerged, exacerbating threats to human safety. The mouth is then artificially breached, sometimes illegally. Although causeway removal was a key objective in the management plan, upgrading or construction of alternate roads was a first requirement. Other management objectives included guidelines for artificial mouth breaching and restoration of freshwater runoff. Without freshwater inflow the health of the estuary will continue to deteriorate; the system cannot be managed by only focussing on mouth management or other non-natural structures. [ABSTRACT FROM AUTHOR]

Mangrove-dominated estuaries are highly dynamic and productive systems. The Mngazana Estuary occurs in the sub-tropical biogeographical region along the east coast of South Africa. The present study assessed the water column dynamics in the mangrove-dominated estuary by investigating the response of phytoplankton to the variability in environmental factors and nutrient concentrations in 2014–2015. Freshwater flow into the estuary was variable with a peak at 0.90 m3 s−1 and a monthly average flow of approximately 0.86 m3 s−1. Nutrient concentrations showed seasonal fluctuations in the estuary and are likely derived from both allochthonous (terrestrial runoff) and autochthonous (internal cycling) sources. Results indicate significant, yet unpredictable spatial and temporal variability in phytoplankton, with the main channel exhibiting higher phytoplankton biomass (~ 15 µg l−1) compared to the tidal creeks (~ 8 µg l−1) and phytoplankton composition shifting from pronounced diatom presence (summer 2015) along the main channel to dinoflagellate dominance along the tidal creeks (winter 2015). The potential influences of human activities, ranging from livestock presence to road bridge construction during this study are noted. This study confirms the need for further investigation on mangrove nutrient fluxes to understand their role on the water column microalgal dynamics particularly when freshwater abstraction is high. [ABSTRACT FROM AUTHOR]

• Novel methodology for optimum releases from dams to minimize negative impact on fish migration in downstream estuaries. • Identifies requirement of freshwater flow into estuaries based on fish migration velocity. • Effectively uses The Tennant method and T-FDC approaches for estimating Environmental Flow Requirement (EFR) • Uses hydrodynamic models to find the combined effect of river discharge and tidal conditions on flow state in estuary. • Applicability of methodology is illustrated through a case study in India. Dams play a crucial role in meeting growing water demands, generating energy, and flood protection. However, the controlled water release from dams can significantly impact the ecosystem of downstream areas, especially estuaries. This study investigated the effects of Sardar Sarovar Dam (SSD) on downstream flow and estimated the flow requirements necessary to support anadromous fish (in this study Tenualosa ilisha) migration in the Narmada River estuary. The study used hydrodynamic modeling and environmental flow requirement analysis methods to arrive at conclusions. The results indicated that the SSD significantly altered the downstream flow patterns, particularly during the monsoon season. A reduced freshwater flow resulted in increased tidal influence. The study found that the optimal discharge rates for fish migration range from 1500 to 2250 m3/s, and the minimum flow required to prevent tidal dominance in the estuary is 500 m3/s. The study underscores the importance of appropriate freshwater release from dams to preserve downstream ecology, support fish migration, and avoid adverse effects on fishing communities. [ABSTRACT FROM AUTHOR]

Highlights • Revealed an inverse correlation between relative density of crocodiles and salinity. • Everglades restoration will increase relative density. • Improved our understanding of how crocodiles will respond to Everglades restoration. Abstract The status of the American crocodile (Crocodylus acutus) has long been a matter of concern in Everglades National Park (ENP) due to its classification as a federal and state listed species, its recognition as a flagship species, and its function as an ecosystem indicator. Survival and recovery of American crocodiles has been linked with regional hydrological conditions, especially freshwater flow to estuaries, which affect water levels and salinities. We hypothesize that efforts to restore natural function to Everglades ecosystems by improving water delivery into estuaries within ENP will change salinities and water levels which in turn will affect relative density of crocodiles. Monitoring ecological responses of indicator species, such as crocodiles, with respect to hydrologic change is necessary to evaluate ecosystem responses to restoration projects. Our objectives were to monitor trends in crocodile relative density within ENP and to determine influences of salinity on relative density of crocodiles. We examined count data from 12 years of crocodile spotlight surveys in ENP (2004–2015) and used a hierarchical model of relative density that estimated relative density with probability of detection. The mean predicted value for relative density (λ) across all surveys was 2.9 individuals/km (95% CI: 2.0–4.2); relative density was estimated to decrease with increases in salinity. Routes in ENP's Flamingo/Cape Sable area had greater crocodile relative density than routes in the West Lake/Cuthbert Lake area and Northeast Florida Bay areas. These results are consistent with the hypothesis that restored flow and lower salinities will result in an increase in crocodile population size and provide support for the ecosystem management recommendations for crocodiles, which currently are to restore more natural patterns of freshwater flow to Florida Bay. Thus, monitoring relative density of American crocodiles will continue to be an effective indicator of ecological response to ecosystem restoration. [ABSTRACT FROM AUTHOR]

Modification of freshwater flow regimes can impact estuarine ecosystems at the receiving end of these flows. This is particularly true in tropical areas, such as the wet-dry tropics, where major freshwater flows in the wet season may result in prolonged and large-scale flooding, followed by periods of little or no flow during an extended dry season. Estuarine species, such as the commercially important prawn Penaeus merguiensis, have lifecycles adapted to seasonal flows. The potential effects of flow regulation on estimated catches of P. merguiensis were explored using a Bayesian belief network (BBN) model. Probability statements were generated about the ecological implications of different magnitudes of natural flow, as well as three water-extraction scenarios: 1) reduced flow in the wet season; 2) the introduction of flow in the dry season; and 3) the combination of scenario one and two. Thresholds of freshwater flow that affect the likelihood of high prawn catches in the fishery were identified, and the cumulative effects explored. This is the first time that these thresholds have been identified and the model suggests there is little capacity to regulate or modify the freshwater flow without reducing the probability of high prawn catches. [ABSTRACT FROM AUTHOR]

Highlights • Proportion of fish that did not migrate out of rivers during the spawning season were similar between the two systems. • The length of time individuals spent outside of rivers during the spawning season was less in the Shark River than in the Caloosahatchee. • The proportion of fish that did not return to rivers after the spawning season was higher in the Shark River. Abstract The widespread use of electronic tags allows us to ask new questions regarding how and why animal movements vary across ecosystems. Common Snook (Centropomus undecimalis) is a tropical estuarine sportfish that have been well studied throughout the state of Florida, including multiple acoustic telemetry studies. Here, we ask; do the spawning behaviors of Common Snook vary across two Florida coastal rivers that differ considerably along a gradient of anthropogenic change? We tracked Common Snook migrations toward and away from spawning sites using acoustic telemetry in the Shark River (U.S.), and compared those migrations with results from a previously published Common Snook tracking study in the Caloosahatchee River. We found that the proportions of fish that did not migrate out of rivers during the spawning season and presumably skip spawned were similar between the two systems. However, in the Shark River, there was more year-to-year variability in this behavior, likely tied to freshwater flow and floodplain inundation. Second, we found that the length of time individuals spent outside of rivers during the spawning season (i.e. proxy for time spent spawning) was less in the Shark River than in the Caloosahatchee. Last, the proportion of Snook at emigrated and did not return in the Shark River was higher than in the Caloosahatchee. These latter finding could reflect higher straying rates or higher mortality at spawning grounds. Future work should evaluate whether these spatial differences in river migrations are meaningful enough to affect management. This study illustrates that cross-site comparisons can improve confidence in our understanding of life history metrics while also highlighting differences between sites that are worth exploring to gain a better understanding of a species plasticity in adapting to their environment. [ABSTRACT FROM AUTHOR]

Abstract A bay's capacity to buffer fluvial fluxes between the land and sea is sensitive to hydrological changes that can affect its water renewal rates. In Cartagena Bay, Colombia, pollution issues have been associated with freshwater fluxes which are projected to increase in future years. This has led to plans to reduce freshwater flows by constructing upstream hydraulic doors. Given the influence of freshwater discharge on coastal water renewal, it is important to assess how these upstream changes will affect the bay's hydrodynamic processes. This study calibrated the 3D MOHID Water model, configured with a high-resolution mixed vertical discretization to capture the bay's characteristic processes of vertical stratification and mixing. A Lagrangian transport model was used to analyze the flow of passive particle tracers and calculate water renewal time scales. Mean residence times of 3–6 days and flushing times of 10–20 days for canal water were found, while mean residence times of 23–33 days and flushing times of 70–99 days were calculated for the bay's complete water volume. An assessment of future scenarios showed that increases in freshwater runoff would result in faster water renewal in the bay, while plans to decrease freshwater discharge would result in slower water renewal in the bay. It is therefore imperative that any plans for reducing fluvial fluxes into the bay be accompanied by the control of local pollution sources, which are abundant and could worsen the bay's water quality issues should water renewal times become longer. Highlights • 3D hydrodynamic modelling reveals anthropogenic impacts on estuarine water renewal. • Estuarine processes of vertical mixing and stratification show strong seasonality. • Estuarine circulation and water renewal exhibit dependency on freshwater discharge. • Future scenario assessment provides crucial knowledge for coastal mitigation plans. • This modelling approach is applicable to other Caribbean bays with similar issues. [ABSTRACT FROM AUTHOR]

Estuaries, which are coastal bodies of water connecting the riverine and marine environment, are among the most important ecosystems in the world. Saltwater intrusion is the movement of coastal saline water into an estuary, which makes up estuary water, becomes salty due to the mixing of freshwater with saltwater. It has become a serious environmental problem in the Sebou estuary (Morocco) during wet and dry seasons, which has a considerable impact on residential water supply, agricultural water supply as well as urban industrial production. The aim of this paper is to study longitudinal salinity and the vertical salinity stratification under different hydraulic conditions in the Sebou estuary. Field observations were done during May 2014 and February 2016 and were interpolated using a Geographic Information System (GIS). Measurements results show that the vertically averaged salinity during spring tide is higher than that during neap tide. Also, the stratification increases a larger during the low-flow periods than during the high-flow periods in the Sebou estuary. Based on results of the longitudinal and vertical salinity observations and historical data, empirical equations were obtained for estimation of the intrusion length as a function (1) of upstream freshwater discharge and (2) tidal range in the Sebou estuary. Additionally, a correlation equation between the surface salinity and the vertical average salinity in estuary (at Kenitra station) was proposed. Finally, this paper provides simple equations that are useful to provide first estimates of intrusion length and vertical average salinity in the Sebou estuary, which can be obtained by a simple desk study without the use of every measurements day. [ABSTRACT FROM AUTHOR]

Environmental time series have rich information content that is invaluable for measuring and understanding changes over time and guiding policies to manage change. I extracted information from measurements of 10 water‐quality constituents in upper San Francisco Bay from 1975 to 2016, one of the longest observational records in a U.S. estuary. Changes were detected at every time scale captured by monthly sampling. Long‐term trends included increased ammonium (+53%), nitrate + nitrate (+50%), silicate (+14%), Secchi depth (+42%), and decreased chlorophyll a (Chl a) (−74%) and suspended particulate matter (−45%). Changes at the decadal scale included abrupt shifts (Chl a, nitrate + nitrite) and oscillations between shorter trends of increase and decrease (Secchi depth, phosphate). Long‐term trends were not expressed equally across all seasons, and seasonal patterns of change varied across constituents. These examples illustrate key features of environmental variability at the land–sea interface: (1) water‐quality components change continually at time scales from months to decades; (2) patterns of seasonal, multiyear, and multidecadal change are complex and vary across constituents; (3) primary drivers of change are freshwater inflow, the master regulator of estuarine dynamics, and human activities such as river damming, water diversions, wastewater discharge, environmental policies, and species introductions; (4) extracting the full information content of time series requires multiple analyses, each revealing a different layer of insight into how changes develop over time; (5) water‐quality variability is nonstationary, so future changes cannot be forecast reliably; (6) repeated observation is an essential method of Earth system science with applications in the design and performance measures of environmental policies. [ABSTRACT FROM AUTHOR]

Abstract The Yangtze estuary is considered as an important eco-region of China, but it has been heavily disturbed by reclamation projects. We evaluated the effect of freshwater inflow on the self-restoration of macrobenthic diversity in 3-year-old vegetated, gravel, and mudflat habitats of reclaimed seaward intertidal wetlands. There was a significant self-restoration of species richness and diversity in vegetated and gravel habitats with no inflow of freshwater, but there were no significant changes in species diversity in three habitats subjected to freshwater inflow. Significant changes in community structures of macrobenthos occurred in vegetated and gravel habitats from pre- to post-restoration. Analysis of benthic health showed that gravel habitats were under moderate to high levels of disturbance in pre-restoration period. In contrast, there tended to be no evidence of disturbance in vegetated and mudflat habitats during two investigations. Restoration of macrobenthos is best achieved in vegetated habitats with on inflow of freshwater. Highlights • Macrobenthic restoration was assessed in vegetated, gravel and mudflat habitats. • Effect of inflow of freshwater on macrobenthic restoration was analyzed. • Species diversity had a better restoration in vegetated and gravel habitats. • Self-restoration of macrobenthic diversity was disturbed by inflow of freshwater. • Stem density may be the main indicator of variation in macrobenthic diversity. [ABSTRACT FROM AUTHOR]

The south-eastern Baltic Sea is a particular sensitive marine area which includes coastal waters, shallow land-locked lagoons, and freshwater inflow areas. Thus, it requires monitoring and control of core environmental indicators such as suspended matter concentration. Spatial distribution and temporal variability of the total suspended matter concentration in the sea surface layer was analysed using in situ data and moderate resolution imaging spectroradiometer (MODIS) satellite imagery for 2003-2016. Manifestations of the major contributors of terrigenous particles and organic matter penetrating the study area were revealed. [ABSTRACT FROM AUTHOR]

Columnar sediment samples were collected from five representative river inflow areas of Dianchi Lake, China. The vertical distribution of each form of P were tested. Results showed that the concentration of TP in the sediments from areas A, B, C, D and E in the order of D > B > A > C > E, and the average concentration of D, B, A, C and E were 2991, 2064, 1308, 879, and 759 mg•kg−1, respectively. The concentration of Ex-P, Fe/Al-P, Ca-P and Org-P all decreased with increasing depth. The release of Ex-P was significantly related to TP whereas the Fe/Al-P was not significantly related to TP in the samples from areas polluted by domestic sewage. However, the release of Ex-P and Fe/Al-P were both significantly related to TP in the samples from areas polluted by phosphate mining and phosphate fertilizer application. The results of equilibrium P concentration (EPC0) analysis showed that P in the sediments of areas A, D and E were the source of P in Dianchi Lake, and the P in the sediments of areas B and C were in relative equilibrium with the overlying water. [ABSTRACT FROM AUTHOR]

Given projected changes in river flow to coastal regions worldwide due to climate change and increasing human freshwater demands, it is necessary to determine the role hydrology plays in regulating the biogeochemistry of estuaries. A climatic gradient exists along the Texas coast where freshwater inflow balance ranges from hydrologically positive to negative (where evaporation exceeds inflow) within a narrow latitudinal band, providing a natural experiment for examining inflow effects. Four Texas estuaries ranging from mesosaline to hypersaline were studied for 3 yr to determine how hydrological changes alter the biogeochemistry within and among the estuaries. Trends in dissolved inorganic nutrients, chlorophyll, dissolved organic matter, and carbonate chemistry indicated that these estuaries had drastically different biogeochemical signatures. Nutrients and chlorophyll patterns illustrated an emerging paradigm where phytoplankton biomass in positive estuaries is supported by "new" nitrogen from riverine input, while high concentrations of reduced nitrogen (organic, ammonium) allowed for high chlorophyll in the negative estuary. For carbonate chemistry, a positive estuary receiving river input from a limestone‐dominated watershed was well‐buffered under moderate to high freshwater inflow conditions. When weathering products were diluted during high‐flow conditions, there is carbonate undersaturation (for aragonite) and decreases in pH. However, "acidification" was not observed in the negative estuary because evaporation concentrated the dissolved species and increased buffering capacity. Hydrological changes over spatial gradients are analogous to climatic changes over time, meaning climate change forecasts of higher temperatures and decreased precipitation can make the biogeochemistry of fresher estuaries change to the patterns of saltier estuaries. [ABSTRACT FROM AUTHOR]

The planform of estuaries is often described with an ideal shape, which exponentially converges in landward direction. We show how growing topographically forced nonmigratory (i.e., anchored) bars determine the large‐scale estuary planform, which explains the deviations observed in the planform of natural estuaries filled with bars compared to the ideal planform. Experiments were conducted in a 20‐m long, 3‐m‐wide tilting flume, the Metronome. From a narrow, converging channel a self‐formed estuary developed characterized by multiple channels, braided bars, a meandering ebb channel, and an ebb delta. Bars hardly migrated due to the alternating current, but the bar width increased with increasing estuary width. At locations where the estuary width was narrow, major channel confluences were present, while the zones between the confluences were characterized by a higher braiding index, periodically migrating channels, and a relatively large estuary width. At the seaward boundary, confluences were forced in place by the presence of the ebb tidal delta. Between confluences, bars were topographically forced to be nonmigratory. Diversion of flow around forced midchannel bars caused bank erosion. This resulted in a planform shape with a quasiperiodic widening and narrowing at the scale of forced bars. Observations in natural systems show that major confluence locations can also be caused by inherited geology and human engineering, but otherwise the estuary outline is similarly affected by tidal bars. These observations provide a framework for understanding the evolution of tidal bar patterns and the planform shape of the estuary, which has wide implications for navigation, dredging, and ecology. Plain Language Summary: Estuaries, which are also called river mouths, form the transition from rivers to the ocean. A common concept to describe the shape of these tidally influenced estuaries is the concept of an ideal estuary, which describes estuaries as perfectly converging channels. However, in natural systems we often observe planform shapes that deviate from this converging shape. In this article, we show with experiments in a tilting flume that estuaries without any hard boundaries (such as bedrock geology) evolve into planforms with an irregular shape rather than the perfectly converging shape. We identified a mechanism in which sand bars build up in the center of the estuary, after which the flow is diverted around the bars. This process results in local widening where tidal bars initially formed, while the estuary remains relatively narrow at zones between the bars. This mechanism helps to understand the dynamic behavior of estuaries, in which bars form valuable ecological habitat and in which channels provide access to million dollar harbors. Key Points: Quasiperiodic estuary planforms arise from diversion of flow around forced nonmigratory midchannel bars that causes bank erosionSelf‐formed confinements separate zones in which the estuary is wider and bars are more dynamicConfinement spacing scales with bar dimensions and estuary width in experiments and nature [ABSTRACT FROM AUTHOR]

Subtidal flows driven by density gradients affect the tide‐averaged sediment transport in estuaries and, therefore, can influence their long‐term morphodynamic evolution. The three‐dimensional Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport modeling system is applied to numerically analyze the effects of baroclinicity and Earth's rotation on the long‐term morphodynamic evolution of idealized funnel‐shaped estuaries. The morphodynamic evolution in all the analyzed cases reproduced structures identified in many tide‐dominated estuaries: a meandering region in the fluvial‐tidal transition zone, a tidal maximum area close to the head, and a turbidity maxima region in the brackish zone. As the morphology of the estuaries evolved, the tidal propagation (including its asymmetry), the salinity gradient, and the strength of subtidal flows changed, which reflects the strong bathymetric control of these systems. The comparison with barotropic simulations showed that the three‐dimensional structure of the flow (induced by density gradients) has leading order effects on the morphodynamic evolution. Density gradient‐driven subtidal flows (1) promote near‐bed flood dominance and, consequently, the import of sediment into the estuary, (2) accelerate the morphodynamic evolution of the upper/middle estuary, (3) promote a more concave shape of the upper estuary and reduce the ebb‐tidal delta volume, and (4) produce an asymmetric bathymetry and inhibit the formation of alternate bars that would form under barotropic conditions. This latter effect is the consequence of the combined effect of Earth's rotation and baroclinicity. Key Points: Density‐driven flows affect the long‐term morphodynamic evolution of estuaries by enhancing near‐bed flood dominanceThe combined effect of baroclinicity and Coriolis acceleration tends to promote cross‐channel bathymetric asymmetryThree‐dimensional process‐based numerical models accounting for salinity variations resemble bathymetric and hydrodynamic features observed in estuaries [ABSTRACT FROM AUTHOR]

Abstract We investigated the presence of 66 human and veterinary pharmaceuticals from seven therapeutic groups in surface waters of the Tejo estuary. Collection sites covered the entire estuary and included areas near main river inflows and wastewater treatment outfalls, traversing urban, agriculture, aquaculture, and nature reserve areas. Detection of pharmaceuticals was performed via UHPLC-TOF-MS. Pharmaceuticals were found in all sites (32 different compounds in total). Antibiotics, β-blockers, antihypertensives and anti-inflammatories were the most frequently detected (>90%), with variation in concentrations reflecting the multifaceted nature of estuarine surroundings (accumulated site contamination between 15 and 351 ng L−1). Higher concentrations of antidepressant Sertraline (304 ng L−1), non-steroidal anti-inflammatory Diclofenac (51.8 ng L−1), lipid regulator Gemfibrozil (77.0 ng L−1), antihypertensive Ibersartan (161.9 ng L−1) or antibiotic Doxycycline (128.0 ng L−1), among others, though localized may potentially impact key estuarine functions or services. Ultimately, results provide a baseline for regulatory information and future biota evaluations. Highlights • Estuarine waters were screened for 66 human and veterinary pharmaceuticals. • Sampling covered the entire estuary. • 32 different compounds were found in estuarine surface waters. • Pharmaceuticals were detected at all sites but concentrations varied markedly. • Sertraline, Ibersartan, Doxicicline, and Diclofenac had highest concentrations. [ABSTRACT FROM AUTHOR]

The San Francisco Estuary and its upstream watershed have been highly altered by human development following the California Gold Rush in the mid-19th century. In this paper, we explore the inter- and intra-annual variability of freshwater flow to this estuary and the resulting salt intrusion under scenarios that represent pre-development and contemporary conditions. To place this comparison in context with the advent of systematic and accurate flow and salinity measurements in the estuary, we consider an additional "pre-project" scenario that represents early 20th-century water management (circa 1920), after major flood control and reclamation but before the introduction of large water storage, diversion, and export operations. We use an observed climate record that spans 82 years to compare freshwater flow associated with the scenarios' landscape and water use characteristics. Using published relationships between flow and salt intrusion length developed from three-dimensional hydrodynamic modeling, we evaluate the effect of these flow alterations as well as estuarine geometry modifications and historically observed sea level rise on salt intrusion. We conclude that the predevelopment estuary exhibited a more seasonally variable salinity regime, resulting from a more variable inflow regime from the upstream watershed. [ABSTRACT FROM AUTHOR]

The Practical Salinity Scale 1978 (PSS-78)—developed using samples of standard seawater—is widely used as a conductivity-based measure of salinity in oceans and estuaries. Here we evaluate the application of the scale across San Francisco Estuary where salinity reflects a mixture of seawater, riverine inflows, and agricultural return flows from islands within the estuary. We employ an extensive salinity data set that includes measurements of specific electrical conductance (EC) and major ion concentrations to demonstrate the scale's applicability in this estuary. We find the scale to be valid in waters dominated by seawater intrusion as well as in waters dominated by the largest riverine input to the estuary, i.e. the Sacramento River. In these waters, we further find that the scale to be valid well below its recommended lower-bound value of 2.0. However, the scale under-estimates salinity in waters dominated by the San Joaquin River, the second largest riverine input to the estuary. Similarly, the scale under-estimates salinity in agricultural return flows to the estuary. This work shows that a singular PSS-78 relationship cannot be used to accurately compute salinity across the entire estuary from EC measurements—a finding of considerable importance for salinity monitoring and modeling applications. Using data from the estuary, we propose appropriate corrections to the scale for these drainage-influenced waters. We recommend further research into how these modified PSS-78 formulations can be used to more accurately estimate salinity and ionic constituent concentrations in this and other estuaries. [Display omitted] • Practical Salinity Scale 1978 (PSS-78) evaluated across San Francisco Estuary. • PSS-78 found to be valid in seawater dominated portions of estuary and in the major freshwater inflow (Sacramento River) well below the literature-recommended lower bound of 2.0. • PSS-78 found to underpredict salinity in the second major inflow and in agricultural drainage in the estuary. • PSS-78 does not appear to be accurate for estuary-wide salinity estimation. [ABSTRACT FROM AUTHOR]

• This study assessed the effects of altered freshwater input on a fish community. • Patterns of freshwater discharge lacked seasonality. • Predator and prey fish guild metrics were not linearly related. • High freshwater input affected the relationship between fish and their environment. • SEMs can be used to assess restoration success in coastal ecosystems. Freshwater diversions and damming have altered coastal ecosystems greatly, resulting in fundamental changes in salinity, eutrophication, and ensuing shifts in ecological communities. Recently, efforts to restore water delivery and water quality in coastal systems have increased. To explore the impacts of altered freshwater discharge to a subtropical estuary, here Bayesian structural equation models are used to quantify the relationships among water quality properties, nutrients, and predator (sport) and prey (forage) components of the fish community. During periods of relatively low freshwater input, predatory sport fish and lower trophic level forage fish guild dynamics were tightly coupled with water quality and one another. In this low freshwater discharge system, water quality and nutrients were the most strongly linked components of the system. Conversely, during periods of high-water discharge, there was virtually no link between water quality and sport or forage fish guilds, and the sport fish and forage fish guilds became the most tightly coupled components. Ultimately, freshwater discharge disrupted existing linkages within the broader ecosystem. Based on these results, we expect the restoration of low freshwater flow into the estuary would strengthen the relationship between the fish community and the environment. As restoration of estuaries and rivers continues globally, freshwater delivery to downstream systems will be altered. Following the approach presented here, Bayesian structural equation models can be used to generate insight regarding potential system wide shifts following alteration of freshwater delivery, thereby providing a critically important tool for evaluating management strategies. [ABSTRACT FROM AUTHOR]

This research develops an integrated methodology to determine the economic value to anglers of recreational fishery ecosystem services in Everglades National Park that could result from different water management scenarios. The study first used bio-hydrological models to link managed freshwater inflows to indicators of fishery productivity and ecosystem health, then link those models to anglers' willingness-to-pay for various attributes of the recreational fishing experience and monthly fishing effort. This approach allowed us to estimate the foregone economic benefits of failing to meet monthly freshwater delivery targets. The study found that the managed freshwater delivery to the Park had declined substantially over the years and had fallen short of management targets. This shortage in the flow resulted in the decline of biological productivity of recreational fisheries in downstream coastal areas. This decline had in turn contributed to reductions in the overall economic value of recreational ecosystem services enjoyed by anglers. The study estimated the annual value of lost recreational services at $68.81 million. The losses were greater in the months of dry season when the water shortage was higher and the number of anglers fishing also was higher than the levels in wet season. The study also developed conservative estimates of implicit price of water for recreation, which ranged from $11.88 per AF in November to $112.11 per AF in April. The annual average price was $41.54 per AF. Linking anglers' recreational preference directly to a decision variable such as water delivery is a powerful and effective way to make management decision. This methodology has relevant applications to water resource management, serving as useful decision-support metrics, as well as for policy and restoration scenario analysis. [ABSTRACT FROM AUTHOR]

Abstract: Coastal peatlands represent an interface between marine and terrestrial ecosystems; their hydrology is affected by salt and fresh water inflow alike. Previous studies on bog peat have shown that pore water salinity can have an impact on the saturated hydraulic conductivity (Ks) of peat because of chemical pore dilation effects. In this study, we aimed at quantifying the impact of higher salinities (up to 3.5% NaCl) on Ks of fen peat. Two experiments employing a constant‐head upward‐flow permeameter and differing in measurement and salinity change duration were conducted. Additionally, a third experiment to determine the impact of water salinity on the release of dissolved organic carbon (DOC) of the studied peat type was carried out. The results show a decrease of Ks with time, which does not depend on the water salinity but is differently shaped for different peat types. We assume pore clogging due to a conglomerate of physical, chemical, and biological processes, which rather depend on water movement rate and time than on water salinity. However, an increased water salinity did increase the DOC release. We conclude that salinity‐dependent behaviour of Ks is a function of peat chemistry and that for some peat types, salinity may only affect the DOC release without having a pronounced impact on water flow. [ABSTRACT FROM AUTHOR]

AQUATOX, coupled water quality and estuarine hydrodynamic model based on food web, was implemented to simulate the temporal variations of gross primary production (GPP) and ecosystem respiration (R e ) in Haihe estuary of North China, also, the parameter sensitivity was calculated and the main factors of GPP and R e were analyzed. Results showed that GPP ranged from 16 to 1789 mg O 2  m −2  d −1 and R e from 56 to 1083 mg O 2  m −2  d −1 , respectively. GPP and R e exhibited significant temporal variation and both reached the highest level in June, however, GPP and R e reduced in July and August, which could be explained by the increase of turbidity caused by high freshwater inflow discharge in rainy season. Maximum photosynthetic rate of blue-greens was the most sensitive parameter to GPP. Re was strongly related to temperature, the most sensitive parameter was average epilimnetic temperature. A significant negative correlation was found between GPP, R e and freshwater inflow. Furthermore, significant direct and indirect effects of tidal height on GPP were observed by path analysis, increased turbidity induced by tidal action might explain the influence on GPP. [ABSTRACT FROM AUTHOR]

Like fishing, natural regime shifts and human-induced environmental changes are often equally important factors in driving fish stock declines. In the Northwestern Arabian Gulf, many fish stocks are declining, raising questions about the reduction in the flow of Tigris-Euphrates rivers. Here we investigate the relationship between Tigris-Euphrates river flow and the estimated recruitment patterns from assessment models. We found a positive correlation between the estimated finfish recruitment trends and the flow of Tigris-Euphrates rivers. Additionally, the assessment model showed remarkably weak compensation ratio, likely indicating a reduction in the productivity of nursery area of two finfish stocks but not in that of the crustacean stock. Our investigation would be very critical in providing guidelines to the government agencies in the Northwestern Arabian Gulf as well as countries of Tigris-Euphrates basins: to consider the impacts associated with reductions in Tigris-Euphrates river flows on the ecosystem services of the region. [ABSTRACT FROM AUTHOR]

Gravity currents descending along slopes have typically been studied in quiescent environments, despite the fact that in many geophysical settings there is significant externally driven motion. Here we investigate how the head of a gravity current is influenced by interfacial internal waves at the pycnocline of a two-layer ambient water column. Our experimental measurements show that larger amplitude internal waves, interacting with the gravity current, reduce both the mass transport by the gravity current and its thickness. These results suggest that the ambient internal wave field should be considered when estimating transport by gravity currents in geophysical settings with strong internal waves, such as lakes and the coastal ocean. [ABSTRACT FROM AUTHOR]

Quantitative relationships between biological resources and hydrological variables are often needed to fulfill legislative requirements for management of estuaries and coastal waters. We used a 28-year record of blue crab landings to support development of a minimum flow (defined in Florida Statutes as the point at which further withdrawals of freshwater will cause “significant harm” to the resources or ecology) for the Caloosahatchee Estuary in Florida, USA. Annual catch per unit effort (CPUE) was computed from monthly landings of crabs and measures of fishing effort. Both unadjusted and de-trended annual CPUE were significantly correlated with rainfall, freshwater inflow and the Multivariate ENSO Index during the previous year’s dry season. Increases in CPUE from 1 year to the next were positively related to dry season rainfall in the first of the 2 years. The Minimum Flow was intended to protect the resources from an impact taking 2 or more years to recover. Geometric mean functional regressions and Monte Carlo simulations were used to identify the dry season rainfall associated with this level of impact. Both a spectral analysis and the Monte Carlo simulations indicated that this level of rainfall occurred about once every 6 years. The corresponding Minimum Flow ranged from 8.6 to 12.3 m3/s (averaged over the dry season and measured at the headwater control structure of the estuary). This study demonstrated the relationship between hydrologic variables (rainfall, freshwater inflow) and blue crab landings and the usefulness of fisheries catch data in providing the technical basis for developing environmental flow targets. [ABSTRACT FROM AUTHOR]

A 5-km horizontal resolution regional ocean-sea ice-ice shelf model of the Ross Sea is used to examine the effects of changes in wind strength, air temperature, and increased meltwater input on the formation of highsalinity shelf water (HSSW), on-shelf transport and vertical mixing of Circumpolar Deep Water (CDW) and its transformation into modifiedCDW (MCDW), and basal melt of the Ross Ice Shelf (RIS). A 20% increase in wind speed, with no other atmospheric changes, reduced summer sea ice minimum area by 20%, opposite the observed trend of the past three decades. Increased winds with spatially uniform, reduced atmospheric temperatures increased summer sea ice concentrations, on-shelf transport of CDW, vertical mixing of MCDW, HSSW volume, and (albeit small) RIS basal melt. Winds and atmospheric temperatures from the SRES A1B scenario forcing of the MPI ECHAM5 model decreased on-shelf transport of CDW and vertical mixing ofMCDWfor 2046-61 and 2085-2100 relative to the end of the twentieth century. The RIS basal melt increased slightly by 2046-61 (9%) and 2085-2100 (13%). Advection of lower-salinity water onto the continental shelf did not significantly affect sea ice extent for the 2046-61 or 2085-2100 simulations. However, freshening reduces on-shelf transport of CDW, vertical mixing of MCDW, and the volume of HSSW produced. The reduced vertical mixing of MCDW, while partially balanced by the reduced on-shelf transport of CDW, enhances the RIS basal melt rate relative to the twentieth-century simulation for 2046-61 (13%) and 2085-2100 (17%). [ABSTRACT FROM AUTHOR]

As anthropogenic pressures on fresh water continues, a balance must be achieved to ensure estuaries receive sufficient riverine inputs to sustain ecosystem health. With increased demand on freshwater resources upstream, salinities in the coastal systems may increase, providing an opportunity for encroachment or invasion by species with a higher salt tolerance. These events compromise ecosystem function, integrity and sustainability, particularly of native estuarine habitats. Estuarine dependent teleost fish species may be considered appropriate bioindicators of abiotic stressors. While estuarine fish have a wide tolerance of many environmental conditions, each species has its own tolerance range and/or sensitivity, and at specific phases during their life cycle. A statistical analysis was conducted to better understand the relationship of Ictalurus furcatus (blue catfish), Brevoortia patronus (Gulf menhaden) and Lagodon rhomboides (pinfish) to abiotic parameters associated with freshwater inflows (i.e. salinity, nutrients, turbidity, etc). Data for this study were collected in Galveston Bay, Texas (USA) by various state agencies and spanned 124 seasons between 1979 and 2010. Distance based linear models were run to determine correlations between the selected indicator species and water quality parameters known to be related to freshwater inflows. These relationships were visualized with Spearman vectors overlain on principal coordinates analysis plots. Blue catfish and Gulf menhaden displayed a significant inverse correlation with salinity and significant positive correlation with NO 3 − +NO 2 − (μM), DIN:DIP and turbidity (NTU). During periods of drought, blue catfish were found closest to the river mouth, where salinity was lowest when considering other locations within Galveston Bay. Gulf menhaden maintain a wider distribution across the Bay as they have a wider salinity range tolerance. Pinfish display a significant positive correlation with salinity and temperature within Galveston Bay; as bioindicators this species is expected to be more tolerant to higher salinities. These relationships support the potential use of these species as beneficial indicators of changing estuarine conditions, namely freshwater inflows. Further study is essential to determine if these and/or other species can be applied to other bays along the Texas coast as well as other sub-tropical estuarine systems in which they are found. [ABSTRACT FROM AUTHOR]

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Worldwide, estuaries are regarded as amongst the most ecologically threatened ecosystems and are increasingly being impacted by urban development, agricultural activities and reduced freshwater inflow. In this study, we examined the influence of different human activities on the diversity and structure of bacterial communities in the water column and sediment in three distinct, temperate permanently open estuarine systems within the same geographic region of southern Africa. The Kariega system is freshwater-deprived and is considered to be relatively pristine; the Kowie estuary is marine-dominated and impacted by urban development, while the Sundays system is fresh-water dominated and impacted by agricultural activity in its catchment. The bacterial communities in all three systems comprise predominantly heterotrophic species belonging to the Bacteroidetes and Proteobacteria phyla with little overlap between bacterioplankton and benthic bacterial communities at the species level. There was overlap between the operational taxonomic units (OTUs) of the Kowie and Kariega, both marine-influenced estuaries. However, lower species richness in the Kowie, likely reflects the impact of human settlements along the estuary. The dominant OTUs in the Sundays River system were distinct from those of the Kariega and Kowie estuaries with an overall decrease in species richness and evenness. This study provides an important snapshot into the microbial population structures of permanently open temperate estuarine systems and the influence of anthropogenic impacts on bacterial diversity and community structure. [ABSTRACT FROM AUTHOR]